1. Field of the Invention
This invention relates to a method of sterilization and to a substance sterilized by the method. It has general application, but particular application to packaging.
2. Description of the Prior Art
U.S. Pat. No. 4,122,134 discloses a method of and system for packaging commercially sterile foods by placing the sterile food in a plastics container tending towards opacity to visible light radiation and tending towards transparency to ultraviolet radiation. The container is open and the food product is cooled to a temperature slightly above the freezing point of water preparatory to and while in a chamber having an atmosphere of cooled, dry inert, sterile gas, preferably nitrogen, carbon dioxide, or a mixture thereof. In the chamber, the inert gas and food product are irradiated by ultraviolet energy that propagates through the container. The container is sealed in the chamber, whereby the inert atmosphere is maintained on the surface of the food product while stored in the container, at temperatures slightly above the freezing point of water for prolonged time periods.
The purpose of the introduction of the nitrogen and/or carbon dioxide is to remove air and thus oxygen from around the container and from around the food product within the container and also to remove water vapour from around the container and from around the food product, thereby to avoid condensation thereon.
U.S. Pat. No. 3,769,517 discloses apparatus which permits treatment of products with ultraviolet light in a controlled atmosphere and which comprises a chamber having inlet and outlet openings for the passage of a product to be treated, one or more ultra-violet lamps, and at least one gas reservoir within the chamber separated by a foraminous panel. Gas, for example nitrogen, or a nitrogen/oxygen mixture, is introduced into the reservoir, passes through the foraminous panel and over and around the path of travel of a workpiece to be treated.
The nitrogen atmosphere, when employed alone, is again used to remove air and thus oxygen from around the workpiece. The nitrogen also reduces hazards such as the possibility of fire or explosion and the production of ozone during the treatment process.
DE222006A discloses the use of high-energy ionizing radiation for the sterilization of nutrient media for the purpose of assisting the growth of multicellular animal cells and in particular of mammalian cells in vitro.
The use of such ionizing radiation is much more cumbersome than the use of UV radiation, because ionizing radiation requires far greater shielding of the operators from the source of the radiation. An inert gas, such as nitrogen or helium, is used to remove air and thus oxygen from the atmosphere surrounding the nutrient medium in order to avoid degradation of the nutrients in the medium owing to the presence of oxygen. The inert gas also assists in keeping moisture away from the nutrient medium to maintain the nutrient medium completely dry during irradiation.
The patent discloses that a synergistic result is produced by the combination of the steps of irradiating a food product with UV energy for sterilisation and packaging in an inert atmosphere and that certain deleterious colonies of bacteria and possibly other colonies actually decrease in number at the same time that their further growth is inhibited by the combination of the UV radiation and encapsulation or packaging with the inert gas.
Derwent File Supplier No. AN 85-234441(JP60153982) appears to disclose the removal of contamination compounds from the surface of articles by flowing water containing dissolved ozone over the surfaces under the irradiation of UV. The water can contain dissolved ozone or bubbles of ozonized gas.
Where the contamination compounds are organic compounds, they are partially oxidized by the treatment, aldehydes, ketones and carboxylic acids being forces, and removed by the water. Where the contamination compounds are inorganic compounds, they dissolve or disperse in the water and are thus removed.
Ozone is decomposed under the irradiation of UV to form an oxygen radical, an hydroxyl radical and a hydrogen peroxide radical. As such radicals have high activity but short life-time, the treatment generates the radicals on the surfaces themselves and so makes them most effective. The washing effect is further promoted because the reactivity of the contamination compounds can be enhanced by the UV irradiation, thus breaking down the compounds into smaller particles and into more hydrophilic compositions. The washing effect was enhanced with increasing washing time and with increasing ozone dissolved concentration. Use of oxygen gas in ozone generation increased the ozone concentration and enhanced the washing effect.
Such treatment is concerned with removing contamination from the surface of articles and is not concerned with sterilization in which micro-organisms are rendered not-viable.
U.S. Pat. No. 4,309,388 discloses sterilizing apparatus which comprises an enclosure surrounding the top, bottom and sides of a central sterilizing enclosure space and adjacent inlet and outlet enclosure spaces opening horizontally to the exterior of the enclosure at the opposite ends thereof. A continuously moving conveyor carries open-topped glass bottles to be sterilized horizontally through the inlet, sterilizing and outlet enclosure spaces of the enclosure. The sterilizing enclosure space is divided by partition walls into an upper compartment which opens at the bottom thereof into a lower compartment immediately above the path of travel of the open tops of the bottles passing through the lower compartment of the sterilizing enclosure space. Ozone-generating ultraviolet lamps are mounted in the upper compartment and a blower circulates air between the lower and upper compartments and through the opening of the upper compartment into the lower compartment so that ozone generated in the upper compartment is directed into the openings at the tops of the bottles moving in the lower compartment. Ozone decomposition accelerating ultra-violet lamps are positioned in the inlet and outlet enclosure spaces to accelerate decomposition of the ozone in the air escaping to the surrounding atmosphere through the inlet and outlet enclosure spaces.
The UV employed for the purpose of indirectly sterilizing the bottles has a radiation wavelength which is predominantly that in the wavelength band which generates ozone and not in the wavelength band which kills micro-organisms. Although UV radiation predominantly in the wavelength band which kills micro-organisms is employed, it is not employed for the purpose of rendering the micro-organisms non-viable but simply in order to degrade the ozone after its production.
CH387881 discloses a sterilization system in which the objects to be sterilized are subjected in an enclosure to the simultaneous actions of (1) vapors of a bactericidal substance, for example Methanogene, Trioxymethylene and Aldylene, (2) ozone generated in the enclosure by a lamp radiating at a wavelength of 185 nm. and (3) UV radiation of a wavelength of 253.7 nm.
In CH387881, the primary action of the ozone is particularly for the benefit of the operator of the system, in that it deodorizes the vapours from the bactericidal substance and removes their causticity from them, without however modifying their toxicity. This primary action is accomplished by a secondary action in which the effect of the ultraviolet radiation is to produce an activation of the bactericidal action, not only of the ozone but also of the bactericidal vapours.
Patent Abstracts of Japan, Vol.12, No.22 (C-470) (2869) (JP 62-176595A) discloses a method of decomposing a low molecular organic substance by adding ozonated water to a mixture of hydrogen peroxide, and waste water containing the organic substance, while the whole is irradiated with UV. It is not directed to the sterilization of micro-organisms.
In U.S. Pat. No. 4,336,125, a long sheet material to be sterilized is passed through an atmosphere of H.sub.2 O.sub.2 mist of low concentration and of droplet particle size of approximately 10 microns at room temperature for approximately one second and is then irradiated for approximately one second with UV lamps positioned to irradiate opposite surfaces of the sheet material, whereby the material is sterilized as a result of the synergistic effect produced by the combination of these two sterilization steps.